1. Field of the Invention
This invention relates to a field of a charging apparatus for charging a rechargeable battery, such as a Lithium-ion type battery, which is used in a portable electric apparatus, and in particular to a charging apparatus for controlling a supplement of electric current to a rechargeable battery.
2. Discussion of the Background
According to the development of technology, portable electric apparatus, such as a notebook styled computer, are becoming more popular in many fields. As an example, a portable electric apparatus may be powered by an AC power in the operation of a desk top environment, and may be powered by a rechargeable battery in a mobile environment. According to a progression of processing ability of such portable electric apparatus, a consumption of electricity in such portable electric apparatus is increasing. Therefore, it has become wide spread to use a high capacity rechargeable battery device, such as a Lithium-ion type battery, for the portable electric apparatus.
When charging the rechargeable battery, the charging apparatus must be stabilized at an electric voltage for charging. The charging voltage is defined by a type of the rechargeable battery, such as Lithium-ion, NiMH, or NiCd. If the electric voltage is lower than a predetermined range, the charging capacity of the battery is decreased. Furthermore, if the electric voltage is higher than the predetermined range, the charging ability of the battery itself is seriously decreased because of the stress of the charging electric voltage in the battery device. As a result, the rechargeable battery may not be able to output the rated electric voltage from the terminals therein. Especially, the predetermined range of the Lithium-ion type battery is relatively narrow, and if such a Lithium-ion type battery is charged outside of its predetermined range, the maximum input electric current of the battery would be limited and the charging electric voltage must be accurately stabilized.
FIG. 3 shows a circuit diagram of a background charging apparatus for a Lithium-ion type battery. A rated voltage of a battery package 3 is, for example, 4.1 V. In this construction, first, a current stabilized unit 1 is provided for stabilizing an input electric current from an input power supply, and for regulating the highest level of charging current to about 1 A. Furthermore, a voltage stabilized unit 2 is provided for stabilizing a charging electric voltage to terminals of the rechargeable battery package 3, which accuracy is about 4.1.+-.0.05 V.
FIG. 4 describes a charging characteristic of the charging apparatus shown in FIG. 3. As shown in FIG. 4, the terminal voltage of the rechargeable battery package 3 is gradually increased to 4.1 V from its exhausted level. In this status, the charging current is constantly provided at about 1 A. When the terminal voltage reaches about 4.1 V, the voltage stabilized unit 2 provides a constant voltage charging of about 4.1 V. In this status, the charging current would be gradually decreased and concentrated to 0 A.
In a case of using a NiMH battery or a NiCd battery, the completion of the charging could be detected by a rise of temperature of the battery itself. However, in a Lithium-ion type battery, it is difficult to detect the completion of the charging, because a Lithium-ion type battery does not incur a sufficient temperature change. Therefore, in the background charging apparatus as shown in FIG. 3 the completion of the charging is detected based on whether a predetermined period has passed in the constant voltage changing. However, there is some difficulty in detecting a full charge of the battery in the background method.
Additionally, in the background charging apparatus, a constant voltage charging system must be provided in the charging apparatus therein with high accuracy in all of a range of charging current. Therefore, the background charging apparatus requires a consideration of an influence of a loss of electric voltage caused by the connection of a print circuit board inside the charging apparatus. Furthermore, a loss of electric voltage caused by a connection resistance of the battery terminal itself must be considered.
According to a restriction of planning, the charging apparatus must be adjoined to the rechargeable battery package when charging the battery. The charging apparatus may be connected to the battery package with a Kelvin bridge connection so that the terminal voltage of the battery could be accurately monitored. Accordingly, the number of terminals of the battery package may be increased. However, this generates a restriction in planning of the rechargeable battery system. Furthermore, it would be a reason of increasing a cost of the rechargeable battery system. Due to increasing use of the rechargeable battery, it becomes more important to provide a low cost and efficient charging apparatus.